Absorbent materials are used in many different applications, for example in sanitary articles and in wound healing articles. The common requirement is high absorption capacity of the products. There are many different methods available for modifying and optimising the absorption property and capacity of the absorbent material. To this is also connected the property to retain the liquid after absorption, also when the product is put under pressure, which is an important aspect within some fields and applications.
Processes for producing products having a specified pore volume, giving an improved absorption capacity, are known from EP 0 209 884 A2 and EP 0 210 570 A1. These documents describe processes where pore generating particles are added during cross linking process of micro fibrillated cellulose for preparing absorbent retentive pulp. The processes require additional process steps for removing the pore generating particles.
U.S. Pat. No. 5,104,411 describes an absorbent product comprising a cross-linked and freeze dried microfibrillar pulp. The cellulose fibres, originating from, for example chemical wood pulp, are beaten and cross-linked before the step of freeze drying. The chemical pulp is not pre-treated by any oxidation.
US 2003/0073663 A1 describes a process for oxidation of cellulose derivatives, such as methyl cellulose, and ethyl cellulose. The oxidised cellulose derivates are to be used as bio absorbable medical devices. The oxidation is carried out by exposing the cellulose derivative to nitrogen oxide in an inert solvent such as carbon tetrachloride.
EP 1 325 754 A1 describes a wound dressing material comprising oxidised cellulose, wherein the oxidised cellulose is combined with structural protein and in form of sponge or film. The cellulose has preferably an average molecular weight greater than 50 000 and the oxidised cellulose is preferably oxidised regenerated cellulose (ORC).
WO 95/07303 describes a method for oxidising carbohydrates with high specificity, by using hypochlorite in the presence of nitroxyl compounds such as 2,2,6,6-tetramethylpiperidin-1-oxyl (Tempo). The oxidation is performed in an aqueous reaction medium at a pH of between 9 and 13. It is said that the process provided gives increased yield of oxidised carbohydrates, with less side-reactions.
Also EP 1 149 846 describes a process for oxidation of polysaccharides with hypochlorite in presence of, for example, 2,2,6,6-tetramethylpiperidine-1-oxyl. The oxidation process herein described is performed in absence of a bromide catalyst.
In T Saito, A Isogai, Biomacromolecules, 2004, 5, 1983-1989 a method to oxidise cellulose pulp originating from cotton linter with 2,2,6,6-tetramethylpiperidin-1-oxyl (Tempo) is described.
In T Saito, et al., Biomacromolecules, 2006, 5, 1689-1691 a method including oxidation of cellulose pulp originating from cotton linter with 2,2,6,6-tetramethylpiperidin-1-oxyl (Tempo) followed by a homogenisation is described.
In T Saito, et al., Biomacromolecules, 2007, 8, 2485-2491, a method for producing cellulose nanofibres by oxidation of native cellulose with 2,2,6,6-tetramethylpiperidin-1-oxyl (Tempo) is described. This is a method wherein cellulose nanofibres are formed by forcing formation of carboxylic groups under gentle treatment.
WO 2010/071584 describes composite material comprising superabsorbent material and cellulosic nanofibrils.
However, there is a need for cellulosic absorbent materials having high absorption capacity produced by efficient and economical processes, also without using additional components, such as pore generating agents, cross-linkers etc. There is also a demand of environmentally friendly processes.
The present invention fulfils the requirements and needs as are discussed above and provides a process for producing an adsorbent product.